A mother mallard duck with 2 normal ducklings and 1 all black one?

In my backyard, a mother mallard duck was leading her 3 ducklings around by the pond. I noticed one was not like the other two. It was blackish gray in color but was the same size as the other two yellow/brown ducklings. Is this another species of duck that was adopted in by the mother, or a special coloration? These ducks were found in MN.